This invention relates to a machine tool monitor and method for sensing the vibrations of a cutting tool to detect the initial touch to the workpiece, for on-line measurement of workpiece dimensions.
In machining of complex metal parts, such as aircraft engine parts, the dimensions of each part may have to be checked up to nearly 100 times during the machining process. The time required to do this is a significant fraction of the total machining time, and thus has a significant effect on productivity in the machining process. A flexible method of using the tool itself for parts dimensioning reduces the time devoted to this function and increases productivity. Since the tool is capable of damaging the part if its advance is not stopped precisely at the workpiece surface, the tool touch sensing system must be very sensitive and very fast.
Many different off-line and on-line techniques for parts dimensioning have been developed or proposed. The latter includes laser interferometry and the retractable touch trigger probe. A technique using the tool itself to detect tool-workpiece contact by sensing tool vibration is described in U.S. Pat. No. 4,428,055 to J. R. Zurbrick and J. R. Kelley. This invention is an improvement over the foregoing and other prior art systems to reduce false alarms and have greater sensitivity.
In order to detect the initial touch of the advancing cutting tool to the workpiece surface before the tool can mar the workpiece, the tool touch detector must detect the very low vibration signal produced by a light rubbing contact. To avoid false alarms the tool touch sensing system must ignore or reject all other signals. Field tests have shown that operation of some lathes during touch tests produces spiky noise with an amplitude that can exceed that of the light rubbing contact signal by a factor of 100 or more in spite of the use of frequency domain discrimination against the usually low frequency machinery noise sources. The problem is to reject these high amplitude, short duration noise pulses while still retaining prompt detection of the low amplitude continuous light rubbing contact signal.